1. Field of the Invention
This invention relates to a coil structure (hereinafter referred to as a "superconducting magnet") which generates a strong or high magnetic field when an electric current flows through a coil such as a superconducting coil, and more particularly to a superconducting magnet structure which can suitably prevent a change from a superconducting state to a normal state (hereinafter referred to as a "quench") when a dynamic disturbance, such as vibration and an external magnetic field variation or fluctuation, is applied to the superconducting magnet.
2. Description of the Related Art
FIG. 2 shows a conventional superconducting magnet. In FIG. 2, reference numeral 1 denotes a superconducting coil, reference numeral 2 a superconducting-coil container, reference numeral 3 a radiation shield, reference numeral 4 a heat-insulating vacuum vessel, and reference numeral 5 a support member. The superconducting coil 1 is cooled to a liquid helium temperature, and in most cases, generates a strong magnetic field when a constant current flows therethrough. The superconducting-coil container 2 holds the superconducting coil 1 and a cooling medium (liquid helium) therein, and supports the superconducting coil 1 against an electromagnetic force, such as a hoop force, produced in the superconducting coil 1. Therefore, generally, the superconducting-coil container 2 is made of a high-rigidity material such as stainless steel. The radiation shield 3 is provided for the purpose of preventing radiated heat from affecting the liquid helium temperature portion, and is disposed in spaced relation to the superconducting-coil container 2 and the heat-insulating vacuum vessel 4. The radiation shield 3 is made of a material with a good thermal conductivity, such as aluminum. The heat-insulating vacuum vessel 4 maintains a vacuum to thereby shield heat from the exterior. The vacuum vessel 4 is made, for example, of a high-rigidity material such as stainless steel, or a thick material in order to withstand the vacuum force. The support member 5 supports the superconducting-coil container 2, together with the superconducting coil 1, and the radiation shield 3 within the heat-insulating vacuum vessel 4 in a suspended manner. The support member 5 is made of a highly heat-insulating material. In the above liquid helium-cooled superconducting magnet, when the temperature of the superconducting coil 1 rises due to the transfer of external heat, the superconducting state is destroyed or quenched, and the current flowing in the superconducting coil is rapidly attenuated (this phenomenon is known as a "quench".) When the quench occurs, the magnetic field expected to be produced by the superconducting magnet cannot be maintained, and besides in accordance with the attenuation of the superconducting coil current, an eddy current is induced in the associated circumferential parts, such as the radiation shield, which results in a problem that the associated parts are deformed by an electromagnetic force produced by this eddy current. Therefore, in the design of the superconducting coil, it is most important to avoid such heat entry or transfer as to invite the quench, and also to keep the associated parts sound or unaffected even when the quench occurs.
The factors in the entry or transfer of the heat into the superconducting magnet are classified into static factors and dynamic factors. Examples of the static factors are heat radiation and conduction of heat due to the temperature difference between the superconducting magnet and the exterior, and these cannot be avoided under any condition of use of the magnet. An example of the dynamic factor is the generation of heat by an eddy current induced by disturbances such as a relative vibration between the superconducting coil and the associated part (e.g., the radiation shield) and a variation or fluctuation of the external magnetic field. In the superconducting magnet in a stationary condition, the entry of heat due to the above dynamic factor can be neglected.
The above static factors are common to low-temperature devices, and have sufficiently been taken into consideration in the prior art techniques. Namely, the radiation shield 3 and the heat-insulating vacuum vessel 4 are the most basic parts for reducing the entry of heat thereinto due to the heat conduction and the heat radiation. In the conventional superconducting magnets, in addition to using these parts, various means have been adopted in order to further reduce the heat entry and to ensure a mechanical strength thereof when the quenching occurs. For example, in a superconducting magnet disclosed in Japanese Patent Unexamined Publication No. 1-115107, a low-resistivity material is mounted on a superconducting-coil container over an entire circumference of the superconducting-coil container in order to prevent the deformation of a radiation shield due to an electromagnetic force generated when the quenching occurs.
However, in the prior art, sufficient consideration has not been given to the heat entry due to the dynamic factor. The only means heretofore used for dealing with this heat entry have at best been to install the superconducting magnet in a place not subjected to an external magnetic field variation, and to change the position of mounting of devices, such as a cooling pump, so that mechanical vibrations will not be applied to the superconducting magnet. However, with an increasing application of the superconducting magnet, the superconducting magnet is not always used in a stationary condition in which the superconducting magnet is not subjected to dynamic disturbances. Moreover, it can reasonably be expected that the superconducting magnet is used in a free space where an unexpected disturbance may develop. In such a case, it is necessary to take countermeasures against the above-mentioned dynamic factor. The simplest countermeasure that can be considered is to enhance the cooling ability of the superconducting magnet; however, the problems with this countermeasure are an increased size of the magnet and an increased consumption of electric power. Another countermeasure that can be considered is to reduce the eddy current which is the root cause for the heat generation, or to reduce the resistivity of the superconducting-coil container so that the heat generation will not occur even when the eddy current flows. In the prior art technique disclosed in the above Japanese Patent Unexamined Publication No. 1-115107, there is a possibility that the generation of heat by the eddy current flowing in the superconducting-coil container may be reduced because of the provision of the low-resistivity material covering the superconducting-coil container, although this prior art invention is directed to a different object. In this prior art technique, however, there arise the following problems since the resistance of the superconducting-coil container over the entire circumference thereof extending along the superconducting coil is reduced. First, since the eddy current easily flows when exciting the superconducting coil, the current tending to flow through the superconducting coil will be suppressed by the eddy current, and therefore the rise time required for activating the superconducting magnet, as well as the required electric power, will be increased. If the power supply is enhanced in order to provide the increased power and to shorten the rise time, the generation of heat by the eddy current becomes higher, so that the quench will be liable to occur.